Each station on a computer communications network has a unique address, referred to as "my address", or MA. (Hereinafter quote marks ". . . " will be used to set off variable names, and in some instances, a value assigned to a variable in order to clarify the presentation. Also, in order to clarify the presentation, variable names, and sometimes their values are capitalized. Quote marks and capitalization are also used to set off compound nouns.) Also, many stations have a list of address for various purposes. In the event that two stations have the same address, the operation of the network may be destroyed through a failure mode. Also, if one station has an inappropriate entry in a list of the address of stations on the network, the operation of the network also may be destroyed through a failure mode. Failure modes are discussed more fully in the Appendix hereinbelow, and also some of the specific failure modes possible in the Fiber Distributed Data Interface, FDDI, type of communications token ring network are disclosed in the Appendix.
As an example, the "Ringop/Claim Oscillation" failure mode occurs in systems using source address stripping, and may paralyze the communications system. In order to describe the Ringop/Claim Oscillation failure mode several points must be made: 1, a station strips its own frames from a logical ring communications system by comparing the source address field of the frame with its own station address; 2, a station having an address duplicating the address of the transmitting station inappropriately strips from the ring the frames created by the transmitting station; 3, stations lying between the duplicate address station and the transmitting station will receive no frames for the time period that the transmitting station transmits; 4, each station has a timer, referred to as the TVX timer, which causes the station to reinitalize the ring in the event that the station does not receive frames or tokens within the timing interval of the TVX timer; 5, in the event that the transmitting station transmits frames for a time period exceeding the timing interval of the TVX timer, the downstream stations from the duplicate address station will timeout their TVX timers and consequently reinitalize the communications ring. That is, the ring will unexpectedly be reinitalized as a result of source address stripping by a duplicate address station.
A number of tests have been devised to test a computer network for the presence of duplicate station addresses. One such test used in ring configured communications networks is the "Transmit Self Test", and consists of having a station transmit a frame to itself. This test relies on the fact that the ring uses source address stripping to strip frames from the ring. If the frame circumnavigates the ring, then the ring passes the test, and it is assumed that no address duplicating the station's address exists on the ring. If a duplicate address did exist on the ring, it is assumed that the station having the duplicate address would have stripped the frame from the ring because the ring uses source address stripping. However, there are several conditions where this test can give either a missed detection of a duplicate address existing on the ring, or a false alarm that there is a duplicate address. For example, if one station on the ring is broken and simply does not repeat frames correctly the test will fail, and thereby give a false alarm of the existence of a duplicate address of the transmitting station. Also, there is no positive indication of a duplicate address, only a timeout condition of the station transmitting a frame to itself.
A second test used for testing a communications ring for a duplicate address involves two stations, and is called the "Neighbor Response Test". In the Neighbor Response Test an initiating station transmits an inquiry frame with the broadcast destination address and its own address as the source address of the frame. The first station to receive the inquiry frame then transmits a response frame. The response frame has its destination address field containing the address of the initiating station. The first station to receive the inquiry frame is the downstream nearest neighbor of the initiating station. The response frame is then detected and repeated by each intervening station until the response frame reaches the initiating station. The test is based on the fact that as a station detects and repeats a frame, it checks the destination address of the frame against a list of addresses held within the station for frame reception decisions. When the response frame reaches the initiating station, the initiating station checks the indicator bit in order to determine if the indicator bit shows that the initiating station's address was detected in any of the repeating stations. In the event that the indicator bit is "set" when the response frame is received by the initiating station, the ring network fails the test, and it is assumed that the initiating station has its addresses duplicated in one of the repeating stations. However, there are several conditions where the Neighbor Response test can give a false alarm indicating the presence of a duplicate address when there is none, and can also give a missed detection indication that there is no duplicate address of the initiating station on the ring when there is one. A missed detection indication that the ring is clear of duplicate addresses is given by this test when the duplicate address is in a list in a station, where the list is used to source frames such as in a bridge forwarding list, because the destination address of the frame is not tested against such a list; stated succinctly, the initiating station's address is not in the "receive destination address set", also referred to as the "receive DA set", but is used by the duplicate station to strip. A false alarm indication that there is a duplicate address is given by this test in the event that the source address of the frame is in a receive destination address set in the station, but the station does not use this list to transmit frames onto the ring.
No satisfactory test for the presence of duplicate addresses on a ring communications network for computers is available to overcome the false indications that the present tests give.